Preheat oven to 350 F. Coat a 10-by-15-inch baking sheet with some of the cooking spray.

In a medium bowl, mix together the cracker crumbs, melted butter and sugar. Mixture should come together when pressed with your fingers. Pat mixture evenly into bottom of prepared baking sheet. If needed, place plastic wrap over top and use a round object to roll and press evenly.

Bake in preheated oven for 15 minutes or until set. Remove from oven and sprinkle the marshmallows over crust and top with the chocolate pieces. Return to oven and bake for 5 to 8 minutes or until marshmallows and chocolate melt. Remove from oven and cool. Once cool, use a pizza cutter lightly coated with oil to cut into desired-size squares.